(1) Field of the Invention
The present invention relates to a process for the preparation of at least one organic compound by heterogeneously catalyzed partial gas-phase oxidation of at least one organic precursor compound in a reactor loaded with catalyst, in which at least one portion of the components of the reaction gas starting mixture is brought from a low initial pressure to a higher final pressure by means of a compressor.
Here, a complete oxidation of an organic compound with molecular oxygen is understood as meaning that the organic compound is converted under the reactive action of molecular oxygen so that all the carbon contained in the organic compound is converted into oxides of carbon and all the hydrogen contained in the organic compound is converted into oxides of hydrogen. All reactions of an organic compound under the reactive action of molecular oxygen which differ therefrom are summarized here as partial oxidation of an organic compound.
In particular, partial oxidations are to be understood here as meaning those reactions of organic compounds under the reactive action of molecular oxygen in which the organic compound to be partially oxidized contains at least one chemically bonded oxygen atom more after the end of the reaction than before the partial oxidation was carried out.
Here, an ethylenically unsaturated double bond is to be understood as meaning a chemical double bond between two carbon atoms which either may occur singly in the molecule, may be isolated from other multiple bonds or may be conjugated or condensed with other multiple bonds.
In this document, diluent gases which are substantially inert under the conditions of the heterogeneously catalyzed gas-phase partial oxidation should be in particular those diluent gases whose components remain more than 95, preferably more than 99, mol % unchanged under the conditions of the heterogeneously catalyzed partial gas-phase oxidation, when each component is considered by itself.
It is generally known that numerous key chemicals can be produced by heterogeneously catalyzed partial oxidation of different organic precursor compounds with molecular oxygen in the gas phase.
(2) Description of Related Art Including Information Disclosed Under 37 C.F.R. §1.97 and 1.98.
The conversion of n-butane into maleic anhydride, the conversion of propylene into acrolein and/or acrylic acid (cf. for example DE-A 2351151), the conversion of tert-butanol, isobutene, isobutane, isobutyraldehyde or the methyl ether of tert-butanol into methacrolein and/or methacrylic acid (cf. for example DE-A 2526238, EP-A 92097, EP-A 58927, DE-A 4132263, DE-A 4132684 and DE-A 4022212), the conversion of acrolein into acrylic acid, the conversion of methacrolein into methacrylic acid (cf. for example DE-A 2526238), the conversion of butadiene into maleic anhydride (cf. for example DE-A 2106796 and DE-A 1624921), the conversion of n-butane into maleic anhydride (cf. for example GB-A 1464198 and GB-A 1291354), the conversion of ethylene into ethylene oxide or of propylene into propylene oxide (cf. for example DE-AS 1254137, DE-A 2159346, EP-A 372972, WO 89/0710, DE-A 4311608 and Beyer, Lehrbuch derorganischen Chemie, 17th edition (1973), Hirzel Verlag Stuttgart, page 261), the conversion of propylene and/or acrolein into acrylonitrile (cf. for example DE-A 2351151), the conversion of isobutene and/or methacrolein into methacrylonitrile (i.e. in this document, the term partial oxidation is also intended to include partial ammoxidation, i.e. a partial oxidation in the presence of ammonia), the oxidative dehydrogenation of hydrocarbons (cf. for example DE-A 2351151), the conversion of propane into acrylonitrile or into acrolein and/or acrylic acid (cf. for example DE-A 10131297, EP-A 1090684, EP-A 608838, DE-A 10046672, EP-A 529853, WO 01/96270 and DE-A 10028582), etc. may be mentioned by way of example.
The catalysts to be used for such reactions are usually solids.
Particularly frequently, the catalysts to be used are solid oxide materials or noble metals (e.g. Ag). In addition to oxygen, the catalytically active oxide material may contain only one other element or more than one other element (multielement oxide materials). Particularly frequently used catalytically active oxide materials are those which comprise more than one metallic, in particular transition metal, element. In this case, the term multimetal oxide material is used.
Owing to the usually pronounced exothermic character of most heterogeneously catalyzed gas-phase partial oxidations of organic compounds with molecular oxygen, the reactants are usually diluted with a gas which is substantially inert under the conditions of the gas-phase catalytic partial oxidation and which, with its heat capacity, is capable of absorbing the liberated heat of reaction and advantageously influencing the reaction rate.
One of the most frequently used inert diluent gases is molecular nitrogen, which is always automatically used whenever air is used as an oxygen source for the heterogeneously catalyzed gas-phase partial oxidation.
Another inert diluent gas which is often used, owing to its general availability, is steam. Recycle gas is also often used as inert diluent gas (cf. for example EP-A 1180508). Recycle gas is defined as the residual gas which remains when the desired product has been separated off more or less selectively (for example by absorption in a suitable solvent) from the product gas mixture after a one-stage or multistage (in the multistage heterogeneously catalyzed gas-phase partial oxidation of organic compounds, the gas-phase partial oxidation, in contrast to the one-stage heterogeneously catalyzed gas-phase partial oxidation, is carried out not in one reactor but in at least two reactors connected in series, it being possible to add oxidizing agents between successive reactors; the multistage character is used in particular when the partial oxidation takes place in successive steps; in these cases, it is frequently expedient to adapt both the catalyst and the other reaction conditions to the respective reaction step in an optimum manner and to carry out the reaction step in a separate reactor, in a separate reaction stage; however, it can also be used if, for reasons relating to heat removal or further reasons (cf. for example DE-A 19902562), the reaction is spread over a plurality of reactors connected in series; an example of a heterogeneously catalyzed gas-phase partial oxidation frequently carried out in two stages is the partial oxidation of propylene to acrylic acid; the propylene is oxidized to acrolein in the first reaction stage, and the acrolein is oxidized to acrylic acid in the second reaction stage; methacrylic acid production is also frequently carried out in a corresponding manner in two stages, generally starting from isobutene; both abovementioned partial oxidations can, however, also be carried out in one stage (both steps in one reactor) when suitable catalyst loads are used) heterogeneously catalyzed gas-phase partial oxidation of at least one organic compound. As a rule, it predominantly comprises the inert diluent gases used for the partial oxidation and steam usually formed as a byproduct in the partial oxidation and oxides of carbon which are formed by undesired complete secondary oxidation. In some cases, it also contains small amounts of oxygen not consumed in the partial oxidation (residual oxygen) and/or of unconverted organic starting compounds. Usually, only a proportion of the residual gas is used as recycle gas. The remaining amount of residual gas is usually incinerated.
A heterogeneously catalyzed partial gas-phase oxidation is usually carried out over a fixed catalyst bed or in a fluidized catalyst bed.
For this purpose, the reaction gas starting mixture, which substantially comprises the at least one organic precursor compound, molecular oxygen (if required, ammonia in the case of an ammoxidation) and inert diluent gas (including, if required, recycle gas), is as a rule passed through the catalyst load at elevated temperatures (as a rule a few hundred ° C., usually from 100 to 600° C.). The chemical reaction takes place during the period of contact with the catalyst surface.
As stated above in the case of the formation of recycle gas, owing to numerous simultaneous and subsequent reactions taking place in the course of the catalytic gas-phase partial oxidation and owing to the inert diluent gases generally present (in certain circumstances, the at least one organic precursor compound can also act as an inert diluent gas, i.e. when it is present in excess in the reaction gas starting mixture relative to the molecular oxygen contained therein), a heterogeneously catalyzed partial gas-phase oxidation gives not a pure organic desired compound but a reaction gas mixture from which the desired product has to be isolated.
If the region of the gas-phase oxidation forms the actual reaction zone, the product gas mixture is usually fed to a working-up zone for isolating the desired product, in which zone this isolation is effected.
Typically (for example in the case of acrylic acid and in the case of methacrylic acid), the isolation of the desired product from the product gas mixture is effected by extraction, fractional condensation and/or rectification methods in separation columns which contain internals with separation activity and through which the product gas mixture is passed (cf. for example EP-A 1 041 062, EP-A 778 255, EP-A 695 736, DE-A 19 501 325 and EP-A 925 272). As described above, the remaining residual gas is concomitantly used, if required, as recycle gas for diluting the reaction gas starting mixture.
For transporting the reaction mixture through the catalyst load of the heterogeneously catalyzed partial gas-phase oxidation and through the subsequent working-up, a pressure difference between reactor entrance and residual gas exit is necessary.
This pressure difference is usually produced in practice by bringing the reaction gas starting mixture, before it enters the oxidation reactor, to a pressure which is higher than the ambient air pressure. These pressures are typically from 0.2 to 5, frequently from 0.5 to 4.5, often from 1 or 2 to 4, bar gage pressure (excess pressure relative to customary atmospheric pressure). High pressures are required in particular when the gas volume flow to be transported is large (for example in high-load procedures, as described in DE-A 19927624, DE-A 19948248, DE-A 19948241, DE-A 19910508 and DE A 19910506), since the latter also requires a greater pressure drop for transport through the catalyst load, if required intermediate and/or downstream condensers loaded with packings, and the working-up apparatuses, for a given reactor and given working-up apparatus.
While the organic precursor compound to be partially oxidized is frequently stored in liquid form in practice it is generally gaseous at atmospheric pressure and room temperature, simple vaporization is as a rule sufficient for bringing the organic precursor compound to be partially oxidized to the reactor entrance pressure. Steam to be concomitantly used as inert diluent gas is generally available from a very wide range of sources, likewise at sufficient superatmospheric pressure.
As a rule, however, this is not true at least for the oxygen source (e.g. air or oxygen-enriched air), the recycle gas (it usually has a pressure equal to the reactor entrance pressure minus the pressure drop on the way through the oxidation zone and through the working-up zone) and any other inert diluent gases.
In practice, it is therefore usually necessary to bring at least a portion of the components of the reaction gas starting mixture from a low initial pressure to a higher final pressure (generally the reactor entrance pressure) by means of a compressor (cf. for example FIG. 1 of EP-A 990 636).
The compression of these components (e.g. air as the oxygen source and recycle gas as the diluent gas source) can be carried out in spatially separate compressors or in a single compressor.
The portions of the reaction gas starting mixture which originate from various sources and are substantially present at (have substantially been brought to) reactor entrance pressure are transported in separate pipes and are then generally first mixed in a mixer, for example a static mixer (space containing internals which generate turbulence) and then, if necessary, heated to the entrance temperature and then fed to the oxidation reactor (the entrance of the individual gases into the pipe leading to the static mixer is frequently expediently chosen so that the formation of explosive mixtures is avoided (in the case of a partial oxidation of propylene to, for example, acrolein and/or acrylic acid, this entry sequence could expediently be, for example, first recycle gas and/or steam, then crude propene and then air)).
The fact that at least one chemical compound having at least one ethylenically unsaturated double bond is involved in most heterogeneously catalyzed partial gas-phase oxidations (for example in all those cited at the outset) is problematic.
This may be, for example, the organic precursor compound to be partially oxidized (e.g. butadiene, propylene, isobutene, acrolein, methacrolein) or the desired product (e.g. acrylic acid, methacrylic acid, acrylonitrile, methacrylonitrile) or an intermediate.
Particularly recycle gas contains, as a rule, at least one chemical compound having at least one ethylenically unsaturated double bond.
This is not entirely uncritical in that chemical compounds having at least one ethylenically unsaturated double bond generally have a pronounced tendency to undesired free radical polymerization. This tendency to polymerization is further increased in the condensed phase and can be additionally promoted by impurities contained therein, which is why gases which are to be compressed and which have components containing at least one ethylenically unsaturated group are frequently heated before they are fed to a compressor (cf. page 6, lines 5 and 6, of EP-A 990 636), in order substantially to rule out undesired droplet formation during compression.
During continuous operation of a production plant, however, neither such undesired droplet formation nor polymerization-promoting impurities can be fully excluded.
Possible solid and/or liquid impurities in the various components of the reaction gas starting mixture (for example, dust in the air used as an oxygen source), which may be deposited, constitute a further problem area.
In principle, a very wide range of compressor types can be used for compressing gases. Displacement compressors (e.g. reciprocal piston compressors, screw compressors and rotary piston compressors), flow compressors (e.g. turbo compressors, centrifugal compressors, axial compressors and radial compressors) and jet compressors may be mentioned by way of example.